We've just been having a spring clean of my sons room, and found that the external wall on one side of his bedroom is covered in a thin film of mould/damp. This wall is usually hidden behind wardrobe etc, but the thin film is also on the back of the wardrobe which sits about 3 inches from the wall. The film is fairly consistent across the whole wall. We live in a detached house, where the downstairs is cavity, but the upstairs is single brick. The wall in question is the side of the house that is most exposed to the elements (sun on a hot day, driving rain and wind on winters days etc). Is this film likely to be an actual leak through the wall? Or is it more likely a build up of condensation (cold outside the wall, warm inside from heating)?

We obviously want to get this rectified ASAP for our sons health. The wall in question is actually going to be having an extension over it in the not too distant future (probably 2 years), but would like to cure this now if possible. Would one of those internal damp proof paints be any good? Or would somebody recommend something else?

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